1. Field of the Invention
The present invention provides specialized media and related methods whereby a photo-induced, Steady-State, Non-Equilibrium Electron Distribution (“SNED”) of free carriers is developed using Mesoscopic Classical Confinement (“MCC”). The photo-induced SNED of free carriers using MCC finds application across a broad range of technical fields, including as examples, infrared (IR) radiation detection and related imaging systems, light modulation, optical switching, wave-division multiplexing, optical amplifiers, lasers, data memories, and color displays.
2. Description of the Related Art
Advances in materials science have greatly enhanced our ability in recent years to engineer man-made materials with specific physical properties by creating different structural composites of useful materials such as semiconductor and dielectrics. Within the broad class of semiconductor materials, silicon underpins nearly all contemporary microelectronics and will continue to do so for the foreseeable future. However, the indirect band-gap nature of silicon greatly limits its usefulness in applications like optoelectronic and photonic devices. Such applications continue to be dominated by direct band-gap materials such as Gallium-Arsenide (GaAs). As a result, the merger of silicon-based electronics with non-silicon-based photonics has largely required the development of hybrid technologies that are often expensive and complicated to produce.
A myriad of commercial applications continues to drive materials scientists in their pursuit of “silicon based” optoelectronic devices. Desirable all-silicon components include, as examples, lasers and other light emitters, modulators, and photodetectors. These components have application in numerous fields, including infrared imaging.
Infrared imaging is the remote sensing and subsequent display of energy existing in a specific portion of the electromagnetic spectrum. Variations in the displayed image intensity represent apparent temperature variations across an image field. The detected radiation appears to emanate from a target surface, but it actually consists of self-emission, reflected radiation, and atmospheric path radiance. To distinguish a target from its background, the detected radiation must be differentiated from the background radiance.
Practical single element infrared detectors were developed during World War II using a lead salt compound (PbS). The basic technical approach, while much refined, is still in use today.
Since at least the 1970's, the promise of IR imaging has resulted in an intense expenditure of resources to improve IR detection capabilities (i.e., create IR photodetectors having improved detectivity and response time). Applications for IR imaging systems include both military and commercial. Military applications include target acquisition, fire control, reconnaissance, and navigation, among others. Commercial applications extend across a broad reach of fields including civil, environmental, industrial, and medical.
A conventional infrared imaging system typically consists of multiple subsystems. FIG. 1 illustrates a number of these subsystems. IR emitted from a target surface is collected by one or more lenses in an optics subsystem. A mechanical scanner assembly is sometimes incorporated with the optics subsystem to move a detector's instantaneous-field-of-view across an imaging field-of-view. In a scanning system, the output of a single detector may be used to develop an imaged scene's intensity using a rasterized scan line in much the same manner as commercial television. With a staring focal plane array (FPA) there is no scanner and the output of individual detectors in the array provides scene variations. Optical filters associated with the optics subsystem are often used to selectively pass or block certain wavelengths of light.
The photodetector is the heart of every infrared imaging system, because it converts scene radiation into a measurable (or displayable) output signal. Conventionally, amplification and signal processing create an electronic image in which voltage differences represent IR radiation intensity differences resulting from objects in the field-of-view.
Each detector in a staring array or scanning detector based system normally has its own amplifier. Amplifier outputs are multiplexed together and then digitized. The number of channels multiplexed together depends upon the specific system design. However, conventional systems typically have several multiplexers and analog-to-digital (A/D) converters operating in parallel.
Signals are typically digitized because it is relatively easy to manipulate and store digital data. Conventional infrared imaging systems rely heavily on software for gain/level normalization, image enhancement, and scan line interpolation. All of these post-detection, electronic circuits are generalized in FIG. 1. Following amplification, digitization, image reconstruction, and post reconstruction processing, an output signal corresponding to the detected IR image is communicated to control, monitoring, measurement, and/or display apparatus.
The word infrared (along with its abbreviation “IR”) has been given different meanings in accordance with a number of conventional applications. For example, so-called infrared film is commonly sensitive out to wavelengths of about 0.85 μm. Spectral responses to wavelengths greater than 0.7 μm often result in systems being generally labeled “infrared.” Indeed, the term infrared has been used to describe one or more portions of the electromagnetic spectrum ranging from 0.7 μm to 1.0 mm.
In the context of the present invention, the terms infrared and IR are interchangeably used to broadly describe system responses, radiation signals, and/or portions of the electromagnetic spectrum ranging from the near-infrared range beginning at 0.7 μm up through the extreme-infrared range ending about 100 μm. The terms infrared and IR specifically include at least the mid-wavelength infrared (MWIR) band of 2.0 to 7.0 μm and the long-wavelength infrared (LWIR) band of 7.0 to 15.0 μm. Of particular note, the MWIR band contains a first thermal imaging band including wavelengths from between 3.0 to 5.5 μm, and the LWIR band contains a second thermal imaging band including wavelengths from between 8.0 and 14.0 μm. These two thermal imaging bands relate one-for-one with well-understood atmospheric transmission windows that advantageously allow efficient propagation of IR radiation. Most conventional IR applications rely on the detection of IR wavelengths within one or both of these two imaging bands.
Indeed, most conventional IR imaging systems are locked into either the first or the second thermal imaging band by a predetermination of one or more IR wavelengths to be detected. For example, Hg1-xCdxTe (x=0.2) quantum detectors, sodium metal vapor devices, and ZnGeP2 or Ag3AsS3 nonlinear crystal based systems detect IR wavelengths in the second thermal imaging band. In contrast, InSb photodetectors, quantum counters (CW and pulsed), cesium metal vapor detectors, and LiNbO3 nonlinear crystal based systems detect IR wavelengths in the first thermal imaging band. Being locked into either the first or the second thermal imaging band is a significant performance limitation attributable to conventional IR imaging system, regardless of the actual technology enabling IR detection. Multi-color capabilities are highly desirable for advance IR systems. Systems that gather data in separate IR spectral bands can discriminate both absolute temperature and unique signatures of objects in the scene. Multi-band detection also enables advanced color processing algorithms to further improve sensitivity above that of single color devices.
Currently, multi-spectral systems rely on cumbersome imaging techniques that either disperse the optical signal across multiple IR FPAs or use a filter wheel to spectrally discriminate the image focused on a single FPA. Consequently, these approaches are expensive in terms of size, complexity, and cooling requirements.
Infrared imaging systems do not actually sense warmth or cold like a thermometer. Rather, such systems sense electromagnetic radiation in a pre-defined band of interest. The relative breadth or narrowness of this detection band is an important imaging system characteristic.
In its broadest context, a photodetector or photosensor is any device that “converts” incident radiation in a detection band into a measurable or displayable electrical signal. The photo-detection process can be defined as generally comprising two steps: (1) the absorption of incident infrared light to cause a corresponding change in some device parameter (e.g., conductivity, charge capacitance, voltage, temperature, etc.); and (2) translation of the changed parameter into some measurable value (e.g., voltage, current, etc.). Progress in IR detector technology is connected mainly to semiconductor IR detectors, which are included in the class of photon detectors. In this class of detectors the radiation is absorbed within the material by interaction with electrons. The observed electrical output signal results from the changed electronic energy distribution. The photon detectors show a selective wavelength dependence of the response per unit incident radiation power. They exhibit both perfect signal-to-noise performance and a very fast response. But to achieve this, the photon detectors require cryogenic cooling. Cooling requirements are the main obstacle to the more widespread use of IR systems based on semiconductor photodetectors making them bulky, heavy, expensive and inconvenient to use. Depending on the nature of interaction, the class of photon detectors is further sub-divided into different types. The most important are: intrinsic detectors, extrinsic detectors, photo-emissive (metal silicide Schottky barriers) detectors, and quantum well detectors.
The second class of IR detectors is composed of thermal detectors. In a thermal detector the incident radiation is absorbed to change temperature of the material, and the resultant change in some physical properties is used to generate an electrical output. The detector element is suspended on lags, which are connected to the heat sink. Thermal effects are generally wavelength independent; the signal depends upon the radiant power (or its rate of change) but not upon its spectral content. In pyroelectric detectors, change in the internal spontaneous polarization is measured, whereas in the case of bolometers a change in the electrical resistance is measured. In contrast to photon detectors, the thermal detectors typically operate at room temperature. They are usually characterized by modest sensitivity and slow response but they are cheap and easy to use. The greatest utility in IR technology has been found in bolometers, pyroelectric detectors and thermopiles. Until the nineteen-nineties, thermal detectors have been considerably less exploited in commercial and military systems in comparison with photon detectors. The reason for this disparity is that thermal detectors are popularly believed to be rather slow and insensitive in comparison with photon detectors. As a result, the worldwide effort to develop thermal detectors was extremely small relative to that of photon detectors. In the last decade however, it has been shown that extremely good imagery can be obtained from large thermal detector arrays operating un-cooled at television frame rates. The speed of thermal detectors is quite adequate for non-scanned imagers with two-dimensional (2D) detectors. The moderate sensitivity of thermal detectors can be compensated by a large number of elements in 2D electronically scanned arrays.
While manufacturing and operational tradeoffs exist between specific technologies implementing thermal IR detectors, they generally share a common set of positive characteristics:
operation at room temperature, such that no sophisticated, external cooling system is required;
large format feasibility that allows improved detection sensitivity;
relatively low power consumption and ease of maintenance;
a lightweight, compact nature;
high manufacturability (i.e., high yields) using conventional silicon and/or integrated circuit materials and using conventional processing techniques; and
low cost.
The final benefit listed above is probably the most notable. The low cost nature of thermal IR detectors explains their prevalence in low-end commercial applications. That is, conventional thermal IR detectors are relatively simple to manufacture and operate, and are, therefore, well suited to low-end and mid-range applications where cost considerations dominate performance considerations.
This reality is clearly manifest upon consideration of a list of common disadvantages associated with thermal IR detectors, including:
low detectivity (i.e., limited identification range);
low spatial resolution;
slow response time;
low sensitivity, requiring elaborate calibration and costly corrective electronics;
a requirement for vacuum packaging for heat isolation; and,
a frequent additional requirement for some form of thermo-electric cooling to stabilize temperature.
The contrast between thermal IR detectors and photon (or quantum) IR detectors is striking. Conventional implementations for photon detectors include, as examples: silicide Schottky-barrier devices (platinum silicide); InSb (Indium antimonide); HgCdTe (mercury cadmium telluride or MCT); band-gap engineered inter-subband photodetectors: gallium arsenide (GaAs)-based quantum well infrared photodetectors (QWIP), InSb/InAs type II supper-lattices and, quantum dot infrared photodetectors (QDIP).
Each of these technologies yields a photon IR detector exhibiting an output signal related to a number of absorbed photons, as opposed to the actual energy of the photons. Generally, an electrical current is produced in relation to electron/hole transitions between energy states brought about by the process of photon absorption. Hereafter, all IR detectors outputting a signal varying in relation to a number of absorbed photons will be denominated as “photon (or quantum) IR detectors.”Again, while manufacturing and operational tradeoffs exist between specific technologies implementing photon IR detectors, they generally share the following set of positive characteristics, including:
high spatial and thermal resolutions;
high detectivity, thus enabling long identification ranges;
fast response time; and,
detection of polarized light, thereby allowing distinction between natural objects in the background and manmade objects.
Not surprisingly, conventional photon IR detectors dominate high-end commercial and military applications. That is, conventional photon IR detectors provide superior performance and, thus, dominate high-end applications where performance considerations dominate cost considerations.
Disadvantages generally associated with photon IR detectors include:
uniformity issues caused by the required combination of exotic materials;
difficult manufacturing processes including many steps and low device yields;
materials and processing incompatibility with current silicon processing techniques;
a cooling requirement, usually to cryogenic temperatures;
difficult maintenance issues and high power consumption; and,
high cost.
Many of the disadvantages associated with conventional photon IR detectors are notable. Indeed, these disadvantages have thus far largely overwhelmed the remarkable detection performance offered by photon IR detectors in all but the highest-end and most costly applications.
Large format arrays are most difficult to obtain given the low yields and the often non-uniform nature of the individual photon detectors. The exotic composition materials require custom fabrication lines and highly specialized processing techniques. This lack of compatibility with the mature field of silicon-based semiconductor manufacturing, together with the enormous burden (financial, maintenance, and technical) of a providing a sophisticated, external cooling system lead to the implementation of very expensive and often bulky IR detection systems.
From the exemplary lists of enabling technologies noted above, it is clear that great efforts have been made to identify high performance IR detectors that may be implemented at a reasonable price. Recently, great improvements have been made in thermal IR detectors with the development of micro-bolometers. See, for example, Brady et al., Advances in Amorphous Silicon Un-cooled IR Systems, SPIE Conference on Infrared Technology and Applications XXV, Orlando, Fla., SPIE Vol. 3698, April 1999. However, these devices still suffer from a relatively large element size and slow response speeds. Further, complex MEMS (micro-electromechanical) process techniques are implicated in the fabrication of these devices.
Recent strides have also been made in photon IR detectors with the development of improved QWIP devices that are both IR sensitive and manufacturable at relatively low cost. QWIP devices detect wavelengths that extend into the far-infrared, greatly expanding the IR detection range. In addition, QWIP devices offer new functionality features, such as polarization-sensitive detection. Yet, these devices must operate at temperatures significantly less than 80° K to reduce the dark current in order to achieve optimal performance. Low operation temperature is a fundamental limitation for QWIPs based on type III–V semiconductor materials. This is due to the high strength of the longitudinal optical phonons within these materials, which results in a very strong thermal excitation of the electrons. Therefore, such structures are characterized by large dark current and noise. An additional drawback of QWIPs lies in the fact that they cannot detect normally incident light because of the ‘quantum mechanical polarization rule’ that requires an electric field component perpendicular to the layer planes of the quantum well structure. This polarization rule can be met by illuminating the structure via 45° facets—a feasible solution for single-element detectors or linear arrays only. Another method for satisfying the polarization rule is to use diffraction gratings, which are of practical importance in the case of two-dimensional detector arrays. Consequently, QWIPs have low quantum efficiency and require a long integration time for signals to achieve appropriate detectivity.
Recently developed QDIPs offer a number of advantages, including lower dark currents and an inherent sensitivity to normally incident light. QDIPs also operate at higher temperatures. However, growth techniques for quantum dot structures are still in an early research and development state and quantum dot technology is far from maturity. Problems relating to the control of dot density, size, and shape uniformity, as well as process stability and repeatability, still pose serious challenges. All relative promise notwithstanding, QDIPs, like QWIPs, only respond to a single radiation wavelength, or to a narrow spectral band.
Thus, conventional photon IR imaging systems continue to present the basic choice. A low-cost thermal IR imaging system having relatively low performance capabilities would be selected, unless performance requirements are sufficiently high to justify a much costlier imaging system based on photon IR detectors. That is, performance and cost are traded in relation to the anticipated application or budget. Conventional systems are locked into a single thermal imaging band, and very often into a single detection frequency within that band.
This Hobson's choice between performance and cost in the field of IR imaging systems is just one result of the general lack of competent silicon-based optoelectronic devices. Ideally, an all-silicon, un-cooled, photon IR detector would exist and be readily integrated with existing semiconductor electronics. Such an un-cooled, photon IR detector would be capable of detecting a range, or at least a plurality, of IR frequencies. However, silicon-based technologies are widely recognized for their poor performance in optical applications.
The foregoing detailed discussion of conventional IR imaging systems is exemplary of many the unfortunate tradeoffs, deficiencies, and limitations inherent in conventional optical components and electro-optical systems that rely on either low performance, silicon technologies, high-cost specialty materials, or a hybrid of these two general technologies.
Clearly a new and fundamentally different approach is required.